This invention relates to a laminate product, a pulse jet filter bag, and methods for manufacturing a laminate product and a pulse jet filter bag. In particular, this invention relates to a laminate product including a porous expanded polytetrafluoroetheylene membrane layer and a woven fabric including polytetrafluoroetheylene containing yarn layer.
Fabric filter bags are often used for separating particulate impurities from industrial fluid streams. These filters tend to accumulate a particulate cake which is occasionally removed, or the filter is replaced. It is common in the industrial filtration market to characterize the type of filter bag by the method of cleaning. The most common types of cleaning techniques are reverse air, shaker and pulse jet. Reverse air and shaker techniques are considered low energy cleaning techniques. The reverse air technique is a gentle backwash of air on a filter bag which collects dust on the interior. The back wash collapses the bag and fractures dust cake which exits the bottom of the bag to a hopper. Shaker mechanisms clean filter cake that collects on the inside of a bag as well. The top of the bag is attached to an oscillating arm which creates a sinusoidal wave in the bag to dislodge the dust cake. Pulse jet cleaning techniques employs a short pulse of compressed air that enters the interior top portion of the filter. As the pulse cleaning air passes through the venturi it aspirates secondary air and the resulting air mass violently expands the bag and casts off the collected dust cake. The bag will typically snap right back to the cage support and go right back into service collecting particulate.
Of the three cleaning techniques the pulse jet is the most stressful on the filter media. In addition to withstanding the stresses provided during cleaning, the on-line pulse jet filter bag is generally subjected to higher volumetric airflow/cloth area ratios compared with reverse air and shaker filter bags.
Filter media often used in pulse jet filter bags include polytetrafluoroetheylene (PTFE), fiberglass, polyimides, polyesters, acrylics, and polypropylenes. When the pulse jet bag is used to filter high temperature industrial streams including oxidizing agents, acids or bases, there is a tendency for many of the filter medias to fail prematurely. PTFE is advantageous in its ability to withstand an environment including high temperatures and oxidizing agents.
Presently available PTFE filter media for use in pulse jet filter baghouse applications are based upon PTFE needlefelts. For example, U.S. Pat. No. 4,983,434 to Sassa describes a pulse jet filter bag prepared from a laminate of expanded porous polytetrafluoroetheylene membrane and polytetrafluoroetheylene felt reinforced with a woven scrim. Filters prepared from needlefelts of PTFE fiber are available. These felts typically weigh from 20-26 oz/yd2 and are reinforced with woven scrim (2-6 oz/yd2). The felts can be made up of staple fibers (usually 6.7 denier/filament, or 7.4 dtex/filament) which are 2-6 inches in length.
PTFE needlefelts tend to exhibit shrinkage when first exposed to high temperatures, and tend to stretch over time. Because of the shrinkage of PTFE needlefelts, they are often double heat set prior to use.
A laminate product which can be used in filter applications is provided by the present invention. The laminate product includes a first layer comprising porous expanded polytetrafluoroethylene membrane, and a second layer comprising woven fabric of polytetrafluoroethylene containing yarn. The porous expanded polytetrafluoroethylene membrane preferably has an air permeability of at least 0.01 cfm/ft2 at 0.5 inch gauge, and a thickness of greater than about 0.5 mil according to ASTM D579-89 sec. 35. The woven fabric of polytetrafluoroethylene containing yarn preferably has a weight of between about 15 opsy and about 30 opsy. The polytetrafluoroethylene polymer of the first layer and the second layer can both be obtained from silicone treated PTFE.
The first layer and the second layer can be thermally bonded together, or bonded together using an adhesive. Preferable adhesives include fluorinated polymer adhesive, such as fluorinated ethylene propylene (FEP) copolymer, tetrafluoroethylene/perfluoropropylene copolymer, and polyvinylidene difluoride.
The woven fabric is preferably prepared from expanded polytetrafluoroethylene containing yarn having a size of between about 600 denier and about 2,400 denier. Preferably, the weight of the woven fabric is between about 18 opsy and about 23 opsy. The membrane preferably has a thickness of between about 1 mil and about 2.5 mil.
In order to function well in a pulse jet filter bag application, the laminate product should have an air permeability of between about 7 and about 20 cfm/ft2 according to ASTM D737-75; a mullen burst strength of between about 600 and 1,000 lbs/in according to ASTM 3786; a tensile strength of between about 150 and about 500 lbs/in according to ASTM D579-89; a shrinkage of less than about 3% when heated to 500xc2x0 F. for 2 hours; a flex greater than about 500,000 cycles according to ASTM D2176-63T; and an elongation of less than about 8% at 50 lbs. It should be appreciated that the laminate product can be used in both dry filter applications and liquid filter applications. In the case of liquid filter applications, it is preferable that the laminate product have an air permeability of between about 0.01 and about 7 cfm/ft2 according to ASTM D73775.
A pulse jet filter bag is provided by the present invention. The pulse jet filter bag includes the laminate product having a first layer of porous expanded polytetrafluoroethylene membrane and a second layer of woven fabric of polytetrafluoroethylene containing yam, wherein the laminate product is sewn together to provide a filter bag. Preferably, the laminate product is sewn using a feld seam to provide a filter bag which can be used in a pulse jet filter application.
A method for manufacturing a laminate product is provided by the present invention. The method includes steps of providing a first layer of expanded porous polytetrafluoroethylene membrane, providing a second layer of woven fabric of polytetrafluoroethylene containing yarn, and bonding the first layer and the second layer together. The first and second layer can be bonded either thermally at a temperature of at least about 500xc2x0 F., or using an adhesive.